The prior art has evolved various ways of controlling or "pre-empting" vehicle traffic lights to stop traffic at an intersection so that an emergency vehicle may pass unimpeded through the intersection on a priority basis. One technique involves the placement of a special transmitter in each emergency vehicle which is to be allowed priority passage through intersections. The traffic light controllers at each pre-emptable intersection are equipped with a receiver which receives signals transmitted by the transmitter and thereupon actuates the traffic lights to stop the normal flow of traffic. However this technique is relatively expensive and is cumbersome in that personnel in the emergency vehicle must manually actuate the transmitter in order to control the traffic lights.
Traffic light controllers ar pre-emptable intersections have also been equipped with detectors capable of detecting flashing lights (normally special strobe lights) mounted on each emergency vehicle which is to be allowed priority passage through the pre-emptable intersections. In essence, this is similar to the system mentioned in the preceding paragraph, in that the emergency vehicle light replaces the special transmitter. The system does however enjoy something of a cost and utility advantage over the system mentioned in the preceding paragraph, since emergency vehicles are normally equipped with flashing lights which are actuated in emergency situations. However, the cost advantage diminishes if special lights must be provided in order to actuate the detector circuitry which interfaces with the traffic signal controller. Moreover, the inventors believe that such systems are susceptible to false alarm triggering because, so far as the inventors are aware, there are no regulations prohibiting the use of flashing lights on non-emergency vehicles. Accordingly, private individuals driving non-emergency vehicles may disrupt such systems by equipping their vehicles with flashing lights for the express purpose of actuating the detectors which interface with the traffic light controllers.
In the inventors' view a better solution is to devise circuitry capable of detecting the sounds produced by emergency vehicle sirens. There is a clear cost advantage to this approach, in that emergency vehicles are conventionally equipped with sirens (i.e. the emergency vehicles do not need to be equipped with additional special purpose equipment) and a utility advantage in that such sirens are normally activated in emergency situations (i.e. no separate manual actuation of additional special purpose equipment is required). A further advantage is that regulations do exist which prohibit the use of sirens on non-emergency vehicles.
The prior art has evolved a number of circuits for detecting siren sounds. However, the inventors consider these to be problematic in that they are susceptible to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens. The present invention provides a siren detector for detecting siren sounds within a selected frequency band and having superior immunity to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens.